Implantable cardiac devices are well known in the art. They may take the form of implantable defibrillators or cardioverters which treat accelerated rhythms of the heart such as fibrillation or implantable pacemakers which maintain the heart rate above a prescribed limit, such as, for example, to treat a bradycardia. Implantable cardiac devices are also known which incorporate both a pacemaker and a defibrillator.
A pacemaker may be considered to be comprised of two major components. One component is a pulse generator which generates the pacing stimulation pulses and includes the electronic circuitry and the power cell or battery. The other component is the lead, or leads, having electrodes which electrically couple the pacemaker to the heart. A lead may provide both unipolar and bipolar pacing and/or sensing electrode configurations. In the unipolar configuration, the pacing stimulation pulses are applied or intrinsic responses are sensed between a single electrode carried by the lead, in electrical contact with the desired heart chamber, and the pulse generator case. The electrode serves as the cathode (negative pole) and the case serves as the anode (positive pole). In the bipolar configuration, the pacing stimulation pulses are applied or intrinsic responses are sensed between a pair of closely spaced electrodes carried by the lead, in electrical contact with the desired heart chamber, with the most proximal electrode serving as the anode and the most distal electrode serving as the cathode.
Pacemakers deliver pacing pulses to the heart to induce a depolarization and a mechanical contraction of that chamber when the patient's own intrinsic rhythm fails. To this end, pacemakers include sensing circuits that sense cardiac activity for the detection of intrinsic cardiac events such as intrinsic atrial events (P waves) and intrinsic ventricular events (R waves). By monitoring such P waves and/or R waves, the pacemaker circuits are able to determine the intrinsic rhythm of the heart and provide stimulation pacing pulses that force atrial and/or ventricular depolarizations at appropriate times in the cardiac cycle when required to help stabilize the electrical rhythm of the heart.
Pacemakers are described as single-chamber or dual-chamber systems. A single-chamber system stimulates and senses in one chamber of the heart (atrium or ventricle). A dual-chamber system stimulates and/or senses in both chambers of the heart (atrium and ventricle). Dual-chamber systems may typically be programmed to operate in either a dual-chamber mode or a single-chamber mode.
Recently, there has been the introduction of pacing systems that stimulate in corresponding chambers of the heart as, for example, the right ventricle (RV) and left ventricle (LV). These are termed biventricular stimulation devices.
Biventricular pacing has been shown to coordinate contractions of the left and right ventricles, reduce the amount of blood flow that leaks through the mitral valve, and decreases the motion of the septal wall that separates the chambers of the heart. Such motion can affect the quantity of blood that the ventricle can pump out in a single beat.
Biventricular pacing has been found to be particularly advantageous in patients suffering from congestive heart disease because of the improved ability of the left ventricle to fully pump blood from the heart. As a result, patients are able to tolerate greater exertion, have a longer life span, and experience a higher quality of life.
Biatrial pacing has also been suggested to also lend in coordinating contractions of the right and left atria. As used herein, the term corresponding chambers is meant to refer to either the combination of the right and left atria or the combination of the right and left ventricles.
One form of biventricular pacing is referred to as Cardiac Resynchronization Therapy (CRT). It has been shown to have a particular positive effect on patients with heart failure (HF). There are a number of ways CRT may be performed. For example, during an atrial tracking mode (DDD), the AV/PV intervals may be set to be short, a negative hysteresis value can be used to ensure pacing, or a trigger mode may be established to provide a pacing stimulation pulse upon a sensed event. For non-tracking modes (DDI, VVI) a high ventricular pacing rate or triggered pacing may be used.
Triggered pacing offers the patient a more physiologic AV/PV delay (as it is their own intrinsic rate) and ensures high percentage of pacing during times of atrial tachycardia (AT) or atrial fibrillation (AF), a critical time for HF patients to receive CRT therapy. However, for triggered pacing to be truly effective in synchronization, the sensing should come from the left and right ventricles. Additionally, sensing from both of these corresponding chambers will ensure that long conduction delays or premature ventricular contractions (PVCs) do not cause extra harm from pacing into a vulnerable period that may come from sensing from one chamber only. For example, a PVC may occur on the left side and be sensed over 100 ms later in the right side to which the device would elicit a biventricular pace pulse into a potential vulnerable period.
Using a combined sensing configuration (left and right ventricle) however poses a problem with accurate ventricular tachycardia and fibrillation detection. For example, if the patient has a very wide QRS complex (which is very likely with an HF patient), then combined right and left sensing may cause the device to double count, thus delivering a more aggressive therapy than may be necessary, i.e., fibrillation therapy instead of antitachycardia pacing (ATP) therapy, or even an inappropriate therapy.
For devices with independent right and left sensing, this may not pose such a problem. But for devices that have limited sense channels, an approach that could encompass right and left sensing while pacing and right sensing while in a tachycardia would be preferred. The present invention addresses this and other issues.